Fuel cells convert a fuel and an oxidising agent into electricity, heat and water at two spatially separated electrodes. Hydrogen or a hydrogen-rich gas can be used as the fuel and oxygen or air as the oxidising agent. The energy conversion process in the fuel cell is distinguished by particularly high efficiency. For this reason, fuel cells are gaining increasing importance as an alternative to conventional combustion engines. Furthermore, they are used in stationary combined-heat-and-power units (CHP), as well as in portable applications.
The polymer electrolyte membrane fuel cell (PEMFC) and the direct methanol fuel cell (DMFC, a variation of the PEMFC, powered directly by methanol instead of hydrogen) are suitable for use as energy converters thanks to their compact construction, their power density and high efficiency. The technology of fuel cells is broadly described in the literature, see for example K. Kordesch and G. Simader, “Fuel Cells and its Applications,” VCH Verlag Chemie, Weinheim (Germany) 1996.
In the following section, the technical terms used in the present patent application are described in greater detail:
A catalyst-coated membrane (hereinafter abbreviated “CCM”) consists of a polymer electrolyte membrane that is provided on both sides with a catalytically active layer. One of the layers takes the form of an anode for the oxidation of hydrogen, and the second layer takes the form of a cathode for the reduction of oxygen. As the CCM consists of three layers (anode catalyst layer, ionomer membrane and cathode catalyst layer), it is often referred to as “three-layer MEA.”
Gas diffusion layers (“GDLs”), sometimes referred to as gas diffusion substrates or backings, are placed onto the anode and cathode layers of the CCM in order to bring the gaseous reaction media (hydrogen and air) to the catalytically active layers and, at the same time, to establish an electrical contact. GDLs are usually carbon-based substrates, such as carbon fibre paper or woven carbon fabric, which are highly porous and allow the reaction gases a good access to the electrodes. Furthermore, they are hydrophobic in order to remove the product water from the fuel cell. GDLs can be coated with a microlayer to improve the contact to the membrane. They can be tailored specifically into anode-type GDLs or cathode-type GDLs, depending on which side they are built into a MEA. Furthermore, they can be coated with a catalyst layer for subsequent lamination to the ionomer membrane. These catalyst-coated GDLs are frequently referred to as “catalyst-coated backings” (abbreviated “CCBs”) or gas diffusion electrodes (“GDEs”).
Generally, two different technologies exist for the production of membrane-electrode-assemblies:
In the first technology (herein called “CCM-technology”) the catalyst layers are applied directly to the ionomer membrane resulting in a catalyst-coated membrane (CCM). This technology is described for example in EP 1 037 295 B1, EP 1 176 652 A2 and other pending applications of the applicant. The advantage of this process is the close, intimate contact of the catalyst layer with the membrane, which is achieved at very mild, benign process conditions. The drawback is that the catalyst-coated membrane (CCM) needs to be assembled separately with two GDLs when making a fuel cell stack.
In the second alternative technology (herein called “CCB-technology,” where CCB stands for catalyst-coated backings), the catalyst layers are applied to the gas diffusion layers (GDLs) first. In a following step, two gas diffusion layers are assembled by means of heat and pressure with the ionomer membrane to yield a five-layer MEA. This process is often referred to as hot-pressing or lamination. High pressures and high temperatures are necessary in this process for lamination of the CCB with the ionomer membrane. This may lead to membrane damage or perforation, particularly if thin ionomer membranes (with less than 40 μm thickness) are used. Furthermore, the catalyst layer on the GDL substrate may be damaged and/or densified, which results in a low catalyst utilisation and a low performance.
In EP 1 042 837 B1, a method for manufacturing of a five-layer MEA is disclosed. The processes described therein are run in a continuous way and are based on co-extrusion, extrusion and/or lamination steps. The processes described in EP 1 042 837 B1 are not applicable to the manufacture of MEA products, which contain an ionomer membrane with selectively coated active electrode areas. Furthermore, no details are given in respect to performance and quality of the MEAs produced according to the claimed processes.
Additionally, in the assembly of a PEMFC or DMFC stack, the three aforementioned components must be combined to form a sandwich structure. In this assembly process, the appropriate GDLs should be placed exactly on top of the catalyst layers on both sides of the CCM to match the active area of the catalyst layers. Furthermore, additional components, e.g., sealing materials, should be applied. Thus, the stack assembly process comprises various alignment steps with different components and is very time-consuming and expensive.
There is a need to develop more efficient means by to make MEAs. The present invention provides a manufacturing process for five-layer MEAs that is straightforward, simple and fast. The process should be based on the use of catalyst-coated membranes (CCMs) and should be easily scaleable to a continuous high-volume manufacturing process.